The "Beachball" as it is often called, is an equivalent of the hourglass in Windows. It basically means that the system is very busy or the app you are currently using has no CPU access. If it lasts for more than a minute, it is likely that the app is being starved, and may never get access to the CPU again. If you don't want to wait, restart. Otherwise, if you can change apps, find another one and close it, that may free up the CPU enough to allow your app to work correctly.
This is all due to developers not creating a good way to prevent these problems, due mainly to the system cost, considering this is "rare" to occur.
If it happens often, consider reseating your ram, reparing permissions, and maybe reinstalling the application in question.
TEG